Biological N(2) fixation is an important part of the marine nitrogen cycle as it provides a source of new nitrogen that can support biological carbon export and sequestration. Research in the past decade has focused on determining the patterns of distribution and abundance of diazotrophs, defining the environmental features leading to these patterns and characterizing the factors that constrain marine N(2) fixation overall. In this Review, we describe how variations in the deposition of iron from dust to different ocean basins affects the limiting nutrient for N(2) fixation and the distribution of different diazotrophic species. However, many questions remain about marine N(2) fixation, including the role of temperature, fixed nitrogen species, CO(2) and physical forcing in controlling N(2) fixation, as well as the potential for heterotrophic N(2) fixation.

Marine picocyanobacteria, comprised of the genera Synechococcus and Prochlorococcus, are the most abundant and widespread primary producers in the ocean. More than 20 genetically distinct clades of marine Synechococcus have been identified, but their physiology and biogeography are not as thoroughly characterized as those of Prochlorococcus. Using clade-specific qPCR primers, we measured the abundance of 10 Synechococcus clades at 92 locations in surface waters of the Atlantic and Pacific Oceans. We found that Synechococcus partition the ocean into four distinct regimes distinguished by temperature, macronutrients and iron availability. Clades I and IV were prevalent in colder, mesotrophic waters; clades II, III and X dominated in the warm, oligotrophic open ocean; clades CRD1 and CRD2 were restricted to sites with low iron availability; and clades XV and XVI were only found in transitional waters at the edges of the other biomes. Overall, clade II was the most ubiquitous clade investigated and was the dominant clade in the largest biome, the oligotrophic open ocean. Co-occurring clades that occupy the same regime belong to distinct evolutionary lineages within Synechococcus, indicating that multiple ecotypes have evolved independently to occupy similar niches and represent examples of parallel evolution. We speculate that parallel evolution of ecotypes may be a common feature of diverse marine microbial communities that contributes to functional redundancy and the potential for resiliency.

Dinitrogen (N2) fixation is recognized as an important input of new nitrogen (N) to the open ocean gyres, contributing to the export of organic matter from surface waters. However, very little N2‐fixation research has focused on the South Atlantic Gyre, where dust deposition of iron (Fe), an important micronutrient for diazotrophs, is seasonally low. Recent modeling efforts suggest that N2‐fixation may in fact be closely coupled to, and greatest in, areas of denitrification, as opposed to the oceanic gyres. One of these areas, the Benguela Upwelling System, lies to the east of the South Atlantic Gyre. In this study we show that N2‐fixation in surface waters across the South Atlantic Gyre was low overall (<1.5 nmol N l−1 d−1) with highest rates seen in or near the Benguela Upwelling System (up to ∼8 nmol N l−1 d−1). Surface water dissolved Fe (dFe) concentrations were very low in the gyre (∼0.3 nM or lower), while soluble reactive phosphorus (SRP) concentrations were relatively high (∼0.15 μM). N2‐fixation rates across the entire sampling area were significantly positively correlated to dFe, but also to SRP and NO3−. Thus, high NO3− concentrations did not exclude N2‐fixation in the upwelling region, which provides evidence that N2‐fixation may be occurring in previously unrecognized waters, specifically near denitrification zones. However the gene encoding for a nitrogenase component (nifH) was not detected from known diazotrophs at some stations in or near the upwelling where N2‐fixation was greatest, suggesting the presence of unknown diazotrophs in these waters.

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Nitrogen fixing organisms such as Trichodesmium spp. are abundant in the oligotrophic tropical North Atlantic Ocean, where microplankton (including other diazotrophs) are more likely to be phosphorus (P) than nitrogen (N) limited. Thus, understanding the ability of different functional groups in the plankton to compete for P in this area is important for understanding their relative success. The uptake of phosphate by Trichodesmium spp. colonies and bulk water plankton was measured using 33 PO 4 3 -over a range of concentrations, and kinetic parameters were determined. Nano-and pico-plankton present in bulk water samples have a K s that is nearly 30 times lower than that of Trichodesmium spp. While chl a-normalized alkaline phosphatase activity (APA) in bulk water was an order of magnitude greater than in Trichodesmium spp., Trichodesmium spp. contributes substantially to total APA in the water. Trichodesmium spp. is outcompeted for dissolved inorganic P (DIP), but colonies can satisfy their P needs by supplementing DIP uptake with P cleaved from dissolved organic P (DOP) via alkaline phosphatase.KEY WORDS: Phosphorus · Phosphate · Nitrogen fixation · Trichodesmium · North Atlantic Resale or republication not permitted without written consent of the publisher